In recent years, applications for hydroforming have been growing—particularly in the field of auto parts. The advantages of hydroforming are that it is possible to form an auto part, which used to be made from several press-formed parts, from a single metal tube, that is, combine parts and thereby reduce costs, and reduce the number of welding locations and thereby lighten the weight.
However, the metal tube used as a material is generally uniform in cross-section, so a shape with a large expansion rate (ratio to circumferential length of tube of circumferential length after hydroforming) was difficult to work.
Further, the difficulty of hydroforming is not only affected by the expansion rate, but is also affected by the cross-sectional shape or presence of any bending. In particular, the length of the location expanded has a large effect.
For example, with the T-shape such as in FIG. 1(a), the expanded length is short, so working is easily possible even with a 1.6 or more large expansion rate. As opposed to this, with a shape with a long expanded location such as in FIG. 1(b), working is difficult even if the expansion rate is not that large.
In hydroforming of a long expanded location, unless applying a considerably large axial pushing action, the tube will become thin in wall thickness and end up cracking, but the larger the axial pushing action, the easier the tube will buckle or wrinkle in the tube axial direction.
Further, a long expanded location means that in that region, in the initial state, the metal tube and the mold will not yet be in contact, so buckling or wrinkles will occur more easily.
So far as the inventors know, in the region of an expansion rate of 1.35 or more, hydroforming to 3.5 times or more the outside diameter of the original metal tube is not seen.
In general, to prevent buckling or wrinkles in the hydroforming, it is important to test different load paths of internal pressure and axial pushing action (hereinafter referred to as simple a “load path”) by trial and error to find the suitable load path.
A general example of the load path is shown in FIG. 2. First, it is comprised of stage 1 of raising only the internal pressure (to seal the tube ends, sometimes slight axial pushing actions are also given), stage 2 of applying the internal pressure and axial pushing actions in a broken line pattern, and stage 3 of raising only the internal pressure for obtaining sharp radii of curvature of the corners (with shapes with no corners, sometimes this is omitted, while to secure a seal of the tube ends, sometimes slight axial pushing actions are also given).
Among these, finding a suitable path for stage 2 consumes the most effort and has relied heavily on the skill of the hydroforming workers.
Patent Document 1 introduces an example of this, but this method is a method of preparing in advance a crack limit line and a wrinkle limit line and selecting a load path between the two limit lines.
However, in actuality, it is difficult to prepare these two limit lines. Usually, a large number of experiments and trial and error in analysis of numerical values are required. Further, the limit lines are often broken lines. If so, the number of parameters for determining the broken lines becomes greater and therefore tremendous labor becomes necessary for the trial and error.
Further, Patent Document 2 proposes a method cyclically changing the internal pressure along with the axial pushing action. For example, this is a method of changing the internal pressure to a square wave (a) or sine wave (b) such as shown in FIG. 3.
This method is proposed as a method for preventing cracking, but later research reports that it is also effective in suppressing wrinkles (see Non-Patent Document 1). However, the load path of this method increases in variables such as the waveform, period, amplitude, etc. compared with the variables in the above-mentioned broken line load path, so finding a suitable load path method for becomes even more difficult.
As a method when hydroforming a shape with a long expanded region, other than the above method of using a load path, there is also the method of specially designing the mold.
For example, Patent Document 3 jointly uses movable molds and a counter to realize expansion in the long region while preventing buckling of the metal tube.
However, the mold structure of this method is extremely complicated, so the mold costs become higher. Further, the items controlled during working are not limited to the internal pressure and axial pushing actions (axial pushing actions by movable molds). Facilities enabling control of the retracted position of the counter also become necessary. Further, since the items controlled increase, finding a suitable load path requires greater skill and trial and error.